Answer:
D. Boxer Rebellion
Step-by-step explanation:
After the Spanish-American War, the United States had a free hand in Central America, South-East Asia, and the Pacific. The boxer rebellion intervention was to maintain an open-door policy in China but only China. The US had territories (Colonies) such as Cuba, Panama, Puerto Rico, Hawaii, Guam, Philippines, etc.
Why it's not the other options:
Military strength was a must-have for Imperial policies.
Economic Beliefs were also a factor as the natural resources of colonies were rich and untouched.
Cultural superiority = Manifest Destiny the "God-given" right that Americans can expand their territory